In automatic screw fastening, a screw is inserted into a screw hole in a work, and a screwdriver fastens the screw. If a member that inserts the screw into the screw hole is the screwdriver itself, fastening can conveniently start just after the insertion. Accordingly, one conventionally proposed method accommodates a screw in a tray on a worktable, picks up the screw engaged with a bit utilizing magnetism and a screwdriver, inserts the screw into a screw hole in a work, and fastens the screw.
This method needs an engagement between the bit of the screwdriver and a recess of the screw in the pickup. The tip of the bit has a convex shape such as a cross and a minus and the recess has a concave shape such as a cross and a minus, but they are not originally aligned with each other and thus cannot be engaged with each other in this state. Therefore, the bit is pressed against the recess and slightly rotated (idled) until the bit is inserted into and engaged with the bit. In addition, this method previously memorizes a position of a center axis of the screw or a position of a center axis of a hole for accommodating the screw, and moves the screwdriver so that the center axis of the bit can accord with the center axis of the screw.
Prior art include Patent Documents 1 and 2:    Patent Document 1: Japanese Laid-Open Patent Publication No. 3-221331.    Patent Document 2: Japanese Laid-Open Utility Model Publication No. 5-60736.
However, the idling causes a deformation or a damage of one of the bit and the screw, and a generation of the abrasion powder. The abrasion of the bit and the screw pollutes the product and causes electric and short through the scattered abrasion powder, and the deformation may hinder highly efficient production through defective screw fastening with a specified torque, an increased number of defective products, and an increased number of tool exchanges.
In addition, even if the screwdriver's movement is controlled so that the center axis of screwdriver's bit can accord with the center axis of the recess of the screw, a positional shift between both center axes on a plane orthogonal to both center axes occurs in actually positioning the bit due to wear-out and changes with time of the motor and the transmission mechanism. When the bit is tried to be engaged with the recess while both center axes shift from each other, the bit rubs on the recess and a problem occurs similar to the idling.